Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their durability, and their ability to generate high torque at low speed. Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require reductant-rich conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions.
One such strategy for the abatement of NOx in the exhaust stream from lean burn engines uses NOx storage reduction (NSR) catalysts, which are also known in the art as “lean NOx traps (LNT).” NSR catalysts contain NOx sorbent materials capable of adsorbing or “trapping” oxides of nitrogen under lean conditions and platinum group metal components to provide the catalyst with oxidation and reduction functions. In operation, the NSR catalyst promotes a series of elementary steps which are depicted below in Equations 1-5. In an oxidizing environment, NO is oxidized to NO2 (Equation 1), which is an important step for NOx storage. At low temperatures, this reaction is typically catalyzed by the platinum group metal component, e.g., a platinum component. The oxidation process does not stop here. Further oxidation of NO2 to nitrate, with incorporation of an atomic oxygen, is also a catalyzed reaction (Equation 2). There is little nitrate formation in absence of the platinum group metal component even when NO2 is used as the NOx source. The platinum group metal component has the dual functions of oxidation and reduction. For its reduction role, the platinum group metal component first catalyzes the release of NOx upon introduction of a reductant, e.g., CO (carbon monoxide), H2 (hydrogen) or HC (hydrocarbon) (Equation 3) to the exhaust. This step may recover some NOx storage sites but does not contribute to any reduction of NOx species. The released NOx is then further reduced to gaseous N2 in a rich environment (Equations 4 and 5). NOx release can be induced by fuel injection even in a net oxidizing environment. However, the efficient reduction of released NOx by H2, CO or HC requires overall net rich conditions. A temperature surge can also trigger NOx release because metal nitrate is less stable at higher temperatures. NOx trap catalysis is a cyclic operation. Metal compounds are believed to undergo a carbonate/nitrate conversion, as a dominant path, during lean/rich operations.
Oxidation of NO to NO2 NO+½O2→NO2  (1)
NOx Storage as Nitrate2NO2+MCO3+½O2→M(NO3)2+CO2  (2)
NOx ReleaseM(NO3)2+2CO→MCO3+NO2+NO+CO2  (3)
NOx Reduction to N2 NO2+CO→NO+CO2  (4)2NO+2CO→N2+2CO2  (5)
In Equations 2 and 3, M represents a divalent metal cation. M can also be a monovalent or trivalent metal compound in which case the equations need to be rebalanced.
While the reduction of NO and NO2 to N2 occurs in the presence of the NSR catalyst during the rich period, it has been observed that ammonia (NH3) can also form as a by-product of a rich pulse regeneration of the NSR catalyst. For example, the reduction of NO with CO and H2O is shown below in equation (6).
Reduction of NO to NH3 2NO+5CO+3H2O→2NH3+5CO2  (6)
It is likely that this NH3 formation reaction happens by a consecutive series of reactions where H2 is first formed by the water gas shift reaction (7) and NH3 is formed by reaction of H2 with NO released from the NSR catalyst.
Water Gas Shift ReactionCO+H2O→H2→CO2  (7)
Reduction of NO to NH3 with H2 2NO+5H2→2NH3+2H2O  (8)
This property of the NSR catalyst mandates that NH3, which is itself a noxious component, must also now be converted to an innocuous species before the exhaust is vented to the atmosphere.
An alternative strategy for the abatement of NOx under development of mobile applications (including treating exhaust from lean burn engines) uses selective catalytic reduction (SCR) catalyst technology. The strategy has been proven effective as applied to stationary sources, e.g., treatment of flue gases. In this strategy, NOx is reduced with a reductant, e.g., NH3, to nitrogen (N2) over an SCR catalyst that is typically composed of base metals. This technology is capable of NOx reduction greater than 90%, thus it represents one of the best approaches for achieving aggressive NOx reduction goals.
Ammonia is one of the most effective reductants for NOx at lean condition using SCR technologies. One of the approaches being investigated for abating NOx in diesel engines (mostly heavy duty diesel vehicles) utilizes urea as a reductant. Urea, which upon hydrolysis produces ammonia, is injected into the exhaust in front of an SCR catalyst in the temperature range 180-600° C. One of the major disadvantages for this technology is the need for an extra large reservoir to house the urea on board the vehicle. Another significant concern is the commitment of operators of these vehicles to replenish the reservoirs with urea as needed, and the requirement of an infrastructure for supplying urea to the operators. Therefore, less burdensome and alternative sources for supplying the reductant NH3 for the SCR treatment of exhaust gases are desirable.
As the conditions that emission treatment systems operate under vary for different vehicles powered by lean burn engines, flexible approaches for the design of emission treatment systems are needed to achieve ever more stringent requirements for NOx abatement. In particular, approaches that account for the effect on NOx storage and NH3 formation during lean and rich periods of operation of altering the NSR catalyst composition offer more reliable and practical pathways to achieving this goal.